The present invention relates to electronic weighing devices. More particularly, the present invention relates to an electronic weighing device having a system to determine the accuracy of the weight measurements made by the weighing device.
Historically, calibration of a weighing device, commonly referred to as scale, consisted of placing a known load onto a load-bearing member of the scale. The weight sensed by the scale was then checked to determine whether the same accurately reflected the load on the load-bearing member, typically referred to as a calibrating sequence. Were a discrepancy found, the scale was adjusted so that the weight sensed conformed to load presented. Otherwise, normal operation of the scale would proceed without the need for an adjustment.
To minimize weighing errors, it has been preferred to periodically enter a calibration sequence during the normal operation of a scale. However, the calibration sequence can be time consuming thereby reducing the throughput of the scale. This may prove problematic in high volume weighing applications when weighing bulk products such as bales of cotton, wheat and the like. In these high throughput operations a historic trade-off has existed between the accuracy of weight measurements and the amount of weight measurements that may be made in a unit of time.
To facilitate calibration, many scales have a built-in calibration weight. U.S. Pat. No. 4,932,486 to Komoto et al. discloses such as scale. The scale in the ""486 patent includes a housing, a weighing means enclosed within the housing, a pan on which an object to be weighed is placed and a space under a top plate thereof. A sensing member has one end connected to the weighing means and projects out of a top wall of the housing. The remaining end of the sensing member is connected to and supports the pan outside of the housing. A calibration weight is arranged in the spaced under the pan. An operating means loads and unloads the calibration weight to and from the sensing member.
U.S. Pat. No. 4,766,965 to Luchinger discloses an apparatus for depositing reference weights in an electronic scale consisting of two lever connected to reciprocate about vertical axes. One end of each of the levers is always in contact with a cam disc attached on a vertically disposed shaft. The remaining end of each lever defines a gliding shoe that may be pushed below a lifting tray. By rotating the cam shaft, the lifting trays cause the reference weights to be deposited by the cam.
U.S. Pat. No. 3,738,439 to Herbert discloses a platform scale having a testing mechanism that includes a calibrated weight for determining the accuracy of the scale. The testing mechanism comprises a plurality of hangers fixed to the underside of the scale platform. The calibrated weight is suspended from the hangers. A lifting mechanism is provided that includes a plurality of bell crank levers driven by jack screws in response to actuation of a reversible electric motor. In this manner, the calibrated weight may be selectively placed in supported engagement with the hangers to calibrate the scale.
A drawback with the prior art is that the built-in calibrated weights typically require a complicated lifting mechanism to selectively place the calibrated weight on a load sensing system of the scale.
What is needed, therefore, is a scale having a built-in calibrated weight that may be selectively placed on a load sensing system of the same while minimizing the complexity of the system employed to move the calibrated load.
Provided is a weighing apparatus and method that features a lifting assembly connected between a calibrated load, having a weight associated therewith, and a frame, to vary an amount of the weight placed on the frame. A load-bearing member is coupled to the frame, and a load sensing system connected to the frame. The load sensing system is connected to sense the variance in the weight of the frame or the load supported by the load-bearing member. One embodiment of the lifting assembly includes a pneumatic system having a piston connected between the frame and the calibrated load. The pneumatic system operates to move the calibrated load between first and second positions. In the first position, the calibrated load is positioned spaced-apart from the frame and rests upon a surface to prevent the frame from being subjected to the mass of the calibrated load. In this position, the mass of the frame is minimized so that the load sensing system does not sense any load on the weighing device. Optimally, when placed in the first position, the load sensing system detects no load on the load-bearing member and the frame. Were a display connected to the load sensing system, the display would reflect a quantitative measurement of zero in the desired units, e.g., pound, kilograms and the like.
In the second position, the calibrated load is positioned proximate to the frame. In this position, the load sensing system would sense a load equal to the amount of a change in the mass, or weight, of the frame. The change in mass or weight of the frame is equivalent to the load placed thereon by positioning the calibrated load in the second position. Although the calibrated load may be of virtually any mass desired, in an exemplary embodiment, the mass of the calibrated load provides a weight of 500 pounds.
The frame may have any cross-section shape desired. In the exemplary embodiment, the frame has a rectangular shape defining four joints. The load sensing system includes a plurality of load sensors, and each of the four joints has one of the plurality of load sensors connected proximate thereto to define four supports for the frame. To increase the accuracy of a measurement of a load placed on the load-bearing member, the four supports may lie in a common plane. Placing the four supports in a common plane reduces side-loading of the frame when a load is placed on the load-bearing member. To further reduce side-loading of the mass associated with the calibrated load, the piston associated with the pneumatic system is connected along an axis of symmetry of the calibrated load. In addition, the calibrated load is connected to the frame so that the plurality of load sensors are symmetrically disposed about the calibrated load. Were a single piston employed, the piston rod would be connected to a centroid of the calibrated load. In the exemplary embodiment two piston are employed each having a rod associated therewith. Each of the rods is connected to the calibrated load at regions spaced-apart, but lying on the axis of symmetry.
The method of operating a weighing device includes subjecting the load-bearing member to a load; sensing the load with the load sensing system, defining a sensed load; verifying operational characteristics of the load sensing system; and calibrating the load sensing system by selectively varying the mass, defining a variance, and sensing the variance with the load sensing system. Usually, the load-bearing member is sequentially subjected to multiple loads. The verification of the operational characteristics of the load sensing system, such as the accuracy of weight measurements, occurs periodically. For example the verification of the operational characteristics of the load sensing system may occur after the load-bearing member has been subjected to a predetermined number of the multiple loads or after a predetermined amount of time has lapsed, or both.
These and other embodiments of the present invention, as well as its advantages and features are described in more detail in conjunction with the text below and attached figures.